The present invention relates generally to a unique development system, and more specifically the present invention is directed to an apparatus and process for charging insulating toner particles. In one embodiment the present invention is directed to an apparatus and process for simultaneously metering and charging insulating toner particles to a positive or negative polarity, by delivering such particles into rubbing contact with a metering/charging means, such as a metering/charging roll. Such a system enables the efficient and effective rapid charging of insulating toner particles, without utilizing carrier particles as commonly employed in the prior art.
The development of images by numerous methods, including electrostatographic means is well known. In such systems toner particles are deposited on an electrostatic latent image contained on an insulating surface, such as selenium, using for example the development methods as described in U.S. Pat. No. 3,618,552, cascade development, U.S. Pat. Nos. 2,874,063, 3,251,706 and 3,357,402, magnetic brush development; U.S. Pat. No. 2,217,776 powder cloud development, and U.S. Pat. No. 3,166,432, touchdown development. The cascade development method and powder cloud development method are especially well adopted for the development of line images common to business documents. Images which contain solid areas are, however, sometimes not faithfully reproduced by these methods, accordingly, magnetic brush development systems are employed to reproduce both lines and solid areas.
Magnetic brush development systems employing two-component developer mixtures comprised of toner particles and carrier particles are thus extensively used in electrophotographic devices, since such systems generally provide for the production of high quality images, including dense solid areas, and also reduce unwanted toner deposition in background areas. Nevertheless, there continues to be problems in the design of simple, inexpensive and efficient two-component systems with long-term stability, which problems are in part caused by the need to generate a triboelectric charge on the toner particles with a desired polarity of sufficient magnitude. Also, in the prior art systems, vigorous developer mixing, which is accomplished in a reservoir or sump using a mixing means such as a paddle-wheel, so as to provide for the continual multiple contacts between toner particles, and carrier particles, is required to quickly charge uncharged toner particles added to the developer mixture. This, continual mixing of the developer composition causes toner smearing on the carrier particles, and consequently irreversible degradation in the ability of the carrier particles to triboelectrically charge the toner particles, such degradation usually resulting in inferior copy quality, including undesirable increased background development. Additionally, the xerographic development properties of two-component developer compositions is dependent on the concentration of toner particles in the developer mixture, therefore in order to maintain stable xerographic development properties during cyclic machine operation, the toner concentration is monitored by a device which controls the rate at which toner particles are dispersed to the developer reservoir. The components and hardware required for such toner concentration control devices adds complexity and cost to two-component systems.
Furthermore, present two-component magnetic brush systems are generally inherently inefficient primarily because only a small amount of toner particles which are transported through the development zone are available for deposition onto the image bearing member, since for example, the magnetic field in the development zone tends to stiffen the developer, thus only those toner particles immediately adjacent to the imaging member are available for development. Should the developer composition in such systems be electrically insulating, an additional limitation is imposed on the development efficiency, as developer particles entering the development zone have a neutral charge, and deposition of charged toner particles onto the imaging member produces a layer of oppositely charged developer which opposes further toner deposition. The utilization of conductive developer compositions, however, improves development efficiency since a net-charged developer layer ordinarily produced by toner deposition onto the imaging member is dissipated by such compositions. In these systems, the developer composition conductivity is a complex function of, for example, the composition of the carrier materials used, the concentration of the toner particles, and the intensity of the magnetic field, therefore achieving high quality development for extended time periods with conductive developer compositions continues to be a problem.
In view of some of the disadvantages of two component systems, there has been considerable efforts directed to designing systems which utilize toner particles only, reference for example, U.S. Pat. No. 2,846,333, which discloses single component developer compositions that are comprised of resins, colorants, and magnetic materials. Generally, these systems eliminate the need for a developer reservoir, a toner dispenser, and toner concentration control means. Most of the single component development systems, especially those systems that are in commercial use, consume conductive toner particles, whereby imagewise toner deposition onto the imaging member is obtained by induction toner charging. The electrostatic transfer of conductive toner particles to plain paper is, however, usually inefficient, since the charge on the toner particles can be reversed by induction charging with plain (conductive) paper. Electrophotographic systems employing conductive single component toner particles, therefore, usually require a special (overcoated) paper to achieve sufficient electrostatic toner transfer.
Additionally, in single component development systems containing conductive toner particles, the control of undesirable background, or background suppression cannot be achieved with electrostatic forces, as the toner particles are inductively charged, and deposited on the image bearing member, which is not the situation in two component development systems, wherein the control of background development is accomplished by an electrostatic force acting on the triboelectrically charged toner particles, causing such particles to be directed away from the image bearing member. Should background suppression be desired in single component systems using conductive toner particles, it is usually obtained by reducing the electrostatic force to near zero, followed by generating a magnetic force, which force is derived from a magnetic field originating from a magnet assembly acting on the conductive toner particles loaded with magnetic particles. Since the magnetic force acting on such toner particles is usually weaker than electrostatic forces, background control with single component systems utilizing conductive toner compositions has not always been satisfactory.
Another disadvantage associated with single component development compositions is that a large amount of magnetic material is necessary, at least about 50 percent in some instances, in order to allow for toner transport, and to provide for proper background control. Accordingly, the optical density of toner particles loaded with magnetic materials, such as magnetite, is sufficiently high to negate the effect of adding other light absorbing materials, such as carbon black. Further, the strongly light absorbing properties of magnetic toner particles precludes the production of colored magnetic toner for use in color imaging systems, by the addition of dyes and pigments.
While improvements have been made in the process, apparatus and materials for the development of latent electrostatic images, there continues to be a need for processes and aparatus which will improve the quality of development, are efficient, simple in design, and economical. In particular, there is a need for a single-component imaging system wherein insulative, non-magnetic, and colored toner particles are appropriately charged, and there is obtained two-component image quality utilizing a single-component development method, apparatus and insulative single-component toner particles so as to enable the efficient electrostatic transfer of such particles to plain paper. Also there is a need for processes and apparatus, where single-component toner particles are metered onto a transporting member, and simultaneously rapidly charged to a desired polarity. Further, there is a need for the provision of an apparatus and process, where background is substantially controlled and eliminated by electrostatic forces, and where the reliability of the system hardware, and materials is increased. Additionally, there continues to be a need for an apparatus and process which will allow the uniform development of both fine lines, and large solid areas of an electrostatic latent image, while obtaining minimum background density.